Recently we have developed a robust Brønsted acid-catalyzed activation of ynamides enabling oxidative coupling of diverse carbon nucleophiles.1 As an extension, we envisioned the oxidation of enynamide that would represent an umpolung of silyl dienol ethers in Mukaiyama alkylation. Employing DMSO as the terminal oxidant, oxidative coupling of enynamides efficiently furnished γ-substituted-α,β-unsaturated imides. Furthermore, not only carbon- but heteronucleophiles, such as alcohol, thiol, hydrazides could be employed in this oxidation protocol. In Hammett analysis, the reaction displayed a linear correlation with σ+ parameters with ρ = -1.3 ~ -3.1, indicating the reaction proceeds through a cationic dienolonium intermediate, and the origin of the remote site-selectivity could be ascribed to the charge-development at the terminal carbon. Eyring analysis led us to identify the formation of keteniminium ion as the rate-determining step.2
1. a) D. V.Patil, S. W. Kim, Q. H. Nguyen, H. Kim, S. Wang, T. Huang, S. Shin, Angew. Chem. Int. Ed. 2017, 56, 3670; b). S.W. Kim, T.W. Um, S. Shin, Chem. Commun. 2017, 53, 2733.
2. Q. H. Nguyen, N. H. Nguyen, H. Kim, Shin, S. manuscript in preparation.